[clang] Disable FTZ/DAZ when compiling shared libraries by default. (PR #80475)

[James Y Knight via cfe-commits]

jyknight wrote:

> I don't see why the current denormal-fp-math setting behavior is a blocking issue for this change

Because this PR as-is causes us to start parsing the "-shared" flag for compilation actions in order to determine which denormal-fp-math setting to use. Users should not pass that to compile actions, and if they do, it should result in a `-Wunused-command-line-argument` diagnostic, NOT a behavior change.

https://github.com/llvm/llvm-project/pull/80475